1. Field of Invention
This invention relates to a nuclear magnetic resonance (hereinafter called "NMR") imaging apparatus which is capable of detecting a particular atomic nucleus distribution in a specimen to be inspected from the outside thereof by using NMR; and more particlarly, to improvements in means for distributing magnetic field of the main magnetic field.
2. Description of Prior Art
In an NMR imaging apparatus, a living body, e.g. a human patient, is placed under a magnetic field. A predetermined pulsewise electromagnetic wave is applied to the body in order to excite only the particular atomic nucleus, which is considered to be an object, in the various atoms which form the body. The atomic nucleus, once excited, recovers again to the initial energy state. At this time, the absorbed energy is also emitted to the outside as an electromagnetic wave. The NMR imaging apparatus detects such emitted magnetic field with a coil. This detected signal is called an NMR signal and includes various kinds of information about the object atomic nucleus. The NMR imaging apparatus analyzes such information and visualizes the detected part of the body as a tomographic image. The tomographic image is helpful in diagnosis and analysis of the body.
Such NMR imaging apparatus is required, from the standpoint of its application purposes, to obtain a high quality image. Uniformity in magnetic field distribution is a major factor for determining the quality of the image. Thus, enhancement of uniformity of magnetic field distribution in the area where a specimen (which is the diagnostic object) is placed, results in improvement of the quality of the image.
The area of uniform domain of magnetic field required for the actual NMR imaging apparatus is as follows. For example, when a specimen is scanned in a plane, such as shown in FIG. 2, the uniform domain required for obtaining tomographic images of the abdomen or the breast is 350 mm in diameter and 200 mm in diameter at the head.
In addition, it is a characteristic of the NMR imaging apparatus to obtain tomographic images of the specimen in desired directions. Accordingly, to insure attainment of such characteristics, it has been considered necessary to generate a domain having uniform magnetic field directed to the entire part of a sphere or round bar having a diameter of 350 mm, as shown in FIG. 3. However, disadvantageously, acquisition of such a wide uniform domain requires static magnetic field coils which are extremely accurate. Also, to maintain such accuracy compels constant adjustment, such as fine adjustment of the static magnetic field coils or adjustment of the values of current applied to the shim coils. Thus, manufacturing and maintenance costs are substantial and leave much to be desired. These disadvantages and deficiencies of the prior art are major problems which limit the practical use of NMR imaging apparatus.